Setting of Distance Relays
Zone 1 of Protection
Distance relays can be classified into phase relay and ground relays. Phase relays are used to protect the transmission line against phase faults (three phase, L-L) and ground relays are used to protect against ground faults (S-L-G, L-L-G). In this lecture, we will learn the ways to set distance relay. Just like an overcurrent relay, a distance relay also has to perform the dual task of primary and back up protection. For example, in fig 22.1, the distance relay has to provide primary protection to line AB and back up protection to lines BC, BD and BE.
The primary protection should be fast and hence preferably it should be done without any intentional time delay, while back up protection should operate if and only if corresponding primary relay fails. In fig 22.1, backs operation of relays. Recall that distance relays are directional relays. Typically, distance relays are provided with multiple zones of protection to meet the stringent selectivity and sensitivity requirements. At least three zones of protection are provided for distance relays. Zone 1 is designated by and zones 2 and 3 by respectively. Zone 1 is meant for protection of the primary line. Typically, it is set to cover 80% of the line length. Zone 1 provides fastest protection because there is no intentional time delay associated with it. Operating time of can be of the order of 1cycle. Zone 1 does not cover the entire length of the primary line because it is difficult to distinguish between faults at all of which are close to bus B. In other words, if a fault is close to a bus, one cannot ascertain if it is on the primary line, bus or on back up line. This is because of the following reasons.
(1)CTs and PTs have limited accuracy. During fault, a CT may undergo partial or complete saturation. The resulting errors in measurement of apparent impedance seen by relay, makes it difficult to determine fault location at the boundary of lines very accurately.
(2)Numerical algorithms may use a specific transmission line model. For example, a transmission line may be modeled as a series R – L circuit and the contribution of distributed shunt capacitance may be neglected. Due to model limitation and because of transients accompanied with the fault, working of numerical algorithm is prone to errors.
(3)With only local measurements, and a small time window, it is difficult to determine fault impedance accurately. For example, if the fault has an impedance (), then the derivations of previous lectures are no more exact. The impedance seen by the relay (fig 22.2) for fault F also depends upon the current contribution from the remote end, thus
(4)There are infeed and outfeed effects associated with working of distance relays. Recall that a distance relaying scheme uses only local voltage and current measurements for a bus and transmission line. Hence, it cannot model infeed or outfeed properly.
Consider the operation of distance relay R1 for fault F close to remote bus on line BC (fig 22.3).
Due to the configuration of generators and loads, we see that
Hence,
______(1)
Thus, we see that the distance relay at R1 does not measure impedance. If there is an equivalent generator source at bus E, then it feeds the fault current. Thus and are approximately in phase. This is known as infeed effect. From equation (1), it is clear that infeed causes an equivalent increase in apparent impedance seen by the relay.
From the relay’s perspective, the fault is pushed beyond its actual location. This itself does not sacrifice selectivity. In other words, relay perceives fault to farther away from than its actual location.
However, if there is an equivalent load at bus E, then IAB and IEB are in phase opposition. This causes an apparent reduction in the impedance seen by the relay . In other words, the relay perceives fault to be at a point closer than its actual location. If this perceived point falls well in the section AB, the relay will operate instantaneously for a fault on the back up line, thereby compromising selectivity. Hence, instantaneous primary protection zone () of distance relay is always set below 100% line impedance. Typically, zone 1 is set to cover 0.8 to 0.9 times the primary line length. In other words, we expect errors in measurements of fault impedance to be within 10-20% accuracy. The remaining portion of the primary line is provided with a time delayed protection known as . The zone 2 protection is delayed at least by the coordination time interval, CTI to give first opportunity to relays to clear a close in fault if it falls into its primary protection zone. Note that, relay in fig 22.3 is immune to infeed or outfeed effect for fault F.
Zone 2 and Zone 3 for Protection
Usually zone 2 is set to 120% of primary line impedance. This provides sufficient margin to account for non-zero fault impedance and other errors in relaying.
Also one should note that also provides back up protection to a part of the adjacent line. One would therefore desire that should be extended to cover as large a portion of adjacent line as possible. Typically, to set to reach 50% of the shortest back up line provided that where ZP and ZB are the positive sequence impedance of primary and the shortest back up line respectively. If the shortest back up line is too short then, it is likely that Zp+1.5 ZB will be less than 1.2Zp. In such a case, is set to 1.2 ZP. Since, back up protection has to be provided to entire length of remote line, a third zone of protection, is used. It is set to cover the farthest (longest) remote lines (BD in fig 22.4(a) for relay acting as back up relay). Since its operation should not interfere with operation of relays, it is set up to operate with a time delay of 2 CTI where CTI is the coordination time interval. The settings of relay on an R-X plane is visualized in fig 22.4(b). The timing diagrams are shown in fig 22.4(c).
FIG 22.4 (a, b, c)
Overlap Problem for
There is a specific reason as to why is not set to reach beyond 50% of the shortest remote line. As shown in fig 22.5, if the reach of of a relay is extended too much, then it can overlap with the Z2 of the relay R3.
Fig 22.5 (a)
Under such a situation, there exists following conflict. If the fault is on line BC (and in Z2 of R3), relay R3 should get the first opportunity to clear the fault. Unfortunately, now both compete to clear the fault. This means that Z2 of the relay R1 has to be further slowed down by CTI. This leads to timing diagram (fig 22.5 (b)).
FIG 22.5 (b)
Thus, it is clear that fault clearing time in 20% region of line AB is delayed a bit too much, thereby degrading performance of Z2 of relay R1. Hence, a conscious effort is made to avoid overlaps of Z2 of relay R1and R3. Setting back zone of to maximum of 120% of primary line impedance or primary line impedance plus 50% of smallest back up impedance usually works out as a good compromise to reach as much of back up lines by without getting into overlap problem. However, under certain condition, when the shortest line to be backed up is too short, it may not be possible to avoid overlap. Similarly, one may encounter overlap problem.
Example
Qn.) Consider a protection system shown in fig 22.6. Identify the primary relays for back up relay.
Ans: Relay not only backup’s line BC but also parallel line AB. Therefore, for relay acting as back up, the primary relays are.
Now assuming that pu impedance of all transmission lines in above fig 22.6 is pu /km.
Determine the setting of zone 1 zone 2 and zone3 relays of .
Ans:
[Because BA is the shortest back up line]
[Because BC is the longest back up line]
This approach for setting of distance relays presented in this fig is known as kilometric approach because the set values of impedances are proportional to lengths. In doing so, we have neglected effect of load currents and as well as the effect of change in operating condition in the system. More accurate settings can be computed by evaluating fault impedance seen by the relay for a fault by using short circuit analysis programs.
Problem of Load Encroachment
Consider the steady state positive sequence model of a transmission line shown in fig 22.7.
Then, it can be shown that apparent impedance seen by relay R is given by,
= ------(2)
Thus from equation (2), we can derive following conclusions;
1. Quadrant of the ZR in R – X plane correspond to the quadrant of apparent power (Sij) in (Pij - Qij) plane.
2. The apparent impedance seen by the relay is proportional to square of the magnitude of bus voltage. If the bus voltage drops say to 0.9pu from 1pu, then ZR reduces to 81% of its value with nominal voltage. Further, if the bus voltage drops say 0.8pu, then the apparent impedance seen by the relay will drop to 64% of its value at 1pu.
3. The apparent impedance seen by the relay is inversely proportional to the apparent power flowing on the line. If the apparent power doubles up, the impedance seen by relay will reduce by 50%.
During peak load conditions, it is quite likely that combined effect of (2) and (3) may reduce the apparent impedance seen by the relay to sufficiently small value so as to fall in characteristic. This is quite likely in case of a relay backing up a very long line. In such a case, impedance setting can be quite large. If the impedance seen by relay due to large loads falls within the zone, then it will pick up and trip the circuit after its time dial setting requirement are met. Under such circumstances, the relay is said to trip on load encroachment. Tripping on load encroachment compromises security and it can even initiate cascade tripping which in turn can lead to black outs. Thus, safeguards have to be provided to prevent tripping on load encroachment. A distinguishing feature of load from faults is that typically, loads have large power factor and lead to with large ratio. In contrast, faults are more or less reactive in nature and the ratio is quite high. Thus, to prevent tripping on load encroachment, the relay characteristic are modified by excluding an area in R – X plane, which corresponds to high power factor. A typical modified characteristic to account for load encroachment is shown in fig 22.7.
The conditions of low value of discussed in (1) and (2) can also arise due to voltage instability or transients associated with electromechanical oscillations of rotors of synchronous machines after a major disturbance like most faults. This can also induce nuisance tripping. Such tripping is known as “tripping on power swings” and it will be studied in the next lecture.